1. Field of the Invention
Embodiments of the present invention relate in general to evaluating well drilling fluids and specifically to testing the effectiveness of the durability of inhibition of inhibitive mud systems on reactive wellbore material.
2. Description of the Related Art
In well drilling operations, drilling mud is used to lubricate the cutting surfaces and carry shale, rock, and other material (“cuttings”) up through the wellbore toward the surface of the earth. As one of skill in the art will appreciate, drilling mud is commonly pumped down through the drill string, exits the drill string near the drill bit, and then returns upward through the annulus between the drill string and the wellbore or casing. In some drilling environments, the cuttings are susceptible to degradation after being cut. In particular, some types of shale can break down from a firm, rock-like material into a fine material. Such disintegration of the shale can make continued drilling more difficult. Inhibitive drilling mud can be used to prevent or delay the disintegration of the drilling mud. The inhibitive drilling mud can include chemical or polymer inhibitors that prevent the shale from disintegrating in the presence of the drilling mud. In some cases, the shale can be further degraded when it contacts non-inhibitive fluids in the wellbore. A non-inhibitive fluid is a fluid that does not inhibit the degradation of shale, and can include non-inhibitive drilling mud and reactive fluids such as water. Some inhibitive mud systems can delay the degradation of the shale in the presence of the other fluids, such as reactive fluids or non-inhibitive mud. The effectiveness of the inhibitors can vary depending on the type of shale and the type of non-inhibitive fluid.
The instantaneous and the long term inhibition of reactive shale formations while drilling is very important to avoid both immediate and time dependent borehole instability problems, especially in the presence of changing fluid environments that are reactive in nature. If the inhibition durability is insufficient, then time dependent borehole instability may occur, even though an inhibitive mud system provided excellent shale stability while drilling. This is due to the fact that the permanency of inhibition of inhibitive mud systems varies significantly due to the variation in the nature, chemistry, inhibition mechanism, inhibition longevity, etc. of the inhibitors used in the formulation of the inhibitive mud systems.
Conventional cuttings dispersion tests can provide relative ranking of the immediate effects of shale inhibition potential of various inhibitive mud systems, but they cannot predict the long term inhibition potential. Therefore, such conventional tests cannot predict the time dependent borehole stability of shale and shale cuttings subject to renewed attack of a non-inhibitive fluid. The conventional shale dispersion test described in API 13I, Section 22, for example, provides relative inhibition potential of inhibitive mud systems, but it is not an indicator of long term inhibition potential of an inhibitive mud system in changing borehole conditions. Therefore, the conventional test cannot detect, for example, that an inhibitive mud providing excellent short term inhibition while drilling may not produce a strong defense mechanism to counter the recurrent attacks of non-inhibitive fluid. The inability to detect long term effect is illustrated by the frequent failure of shale formations that have been drilled safely using an inhibitive mud when the shale comes in contact with a reactive fluid such as water or sea water.
Failures can occur in previously inhibited shale formations when subsequent contact with non-inhibitive fluid induces shale-drilling mud interactions that weaken or destroy the defensive mechanism created by the inhibitors of a previously used inhibitive mud system. Hence, the creation of a strong and durable defensive mechanism to protect the reactive shale/clays from the aging effect and also the recurrent attacks of non-inhibitive fluid are very important for instantaneous and time dependent borehole stability in reactive shale sections of a borehole. As instantaneous and short term inhibition potential of shale inhibitors and inhibitive mud systems are not the indicators of long term inhibition potential of shale inhibitors and inhibitive mud systems, the industry needs a test method and evaluation process to assess the long term inhibition potential of inhibitors and inhibitive mud systems.
It would be helpful to evaluate the permanency or longevity of inhibition, i.e. the inhibition durability limit of inhibitive muds. Unfortunately, there is no API or any other standard laboratory method to evaluate the inhibition durability index (IDI) i.e. longevity of shale inhibition to select the best shale inhibitors or inhibitive mud systems for immediate and long term stabilization of reactive shale formations.